In recent years, volatile organic compounds used in industrial cleaning and hazardous metals contained in industrial effluent have entered soil and caused serious soil contamination.
Various methods are used to treat volatile organic compounds, such as dichloromethane, carbon tetrachloride, dichloroethane, dichloroethylene, trichloroethane, trichloroethylene, tetrachloroethylene, dichloropropane, benzene, chloroform, toluene, and xylene, that enter soil. For example, a method is known wherein contaminated groundwater drawn from contaminated soil is pumped and aerated, and exhaust gas (gas phase) obtained by gas-liquid separation is treated by a technique such as adsorption treatment with activated carbon or pyrolysis treatment.
However, adsorption treatment with activated carbon is problematic in that adsorption capacity soon reaches saturation, and the adsorption effects last for only a short period of time. Pyrolysis treatment is problematic in that a significant amount of energy is required.
Patent Literature (PTL) 1 discloses a method for decomposing volatile organic confounds comprising pumping and aerating contaminated groundwater to obtain gaseous volatile organic compounds, and subjecting the gaseous volatile organic compound to ultraviolet radiation using a photocatalytic material. The technique disclosed in PTL 1 uses a titanium oxide-coated honeycomb-shaped material as a photocatalytic material to increase the reaction area of the photocatalytic material.
However, the technique disclosed in PTL 1 has a drawback in that since fine titanium oxide powder is used as a photocatalytic material while contained in a binder, nearly all of the titanium, oxide particles are embedded in the binder. Low adhesion between the photocatalytic material and the substrate is also a problem of the technique disclosed in PTL 1.
Methods used for treating hazardous heavy metals, such as lead, cadmium, hexavalent chromium, arsenic, mercury, and copper, that enter soil from industrial effluent include a method for chemically chelating heavy metals, a method, for fixing heavy metals by ion exchanging, a method for separating heavy metals by a high-temperature treatment using an incinerator, and the like.
However, these treatment methods not only have problematically high treatment cost, but also have difficulty in recovering low-concentration heavy metals.
Patent Literature (PTL) 2 discloses a method for removing the heavy metal lead using photocatalytic fine particles having iron oxyhydroxide particles dispersed in and supported on a titanium oxide photocatalyst, or using a photocatalytic material having a plate-shaped, porous, or fibrous material coated with the photocatalytic particles.
However, when the photocatalytic fine particles are directly treated, a problematic amount, of time and effort, is required to separate the photocatalytic fine particles from the solution. Furthermore, the method of coating with photocatalytic fine particles has a drawback in that since fine titanium oxide powder is used while contained in a binder, nearly all of the titanium oxide particles are embedded in the binder. Low adhesion between the photocatalytic material and the substrate is also problematic in the technique disclosed in PTL 2.